The present invention relates generally to displays, and more particularly to displays utilizing a gain reflector and a display medium that may be switched between light scattering and non-scattering states.
Visual display devices may utilize liquid crystals. The property of liquid crystals that makes them particularly useful in visual displays of the type of the present invention is the ability of certain liquid crystal materials to transmit light in a strictly aligned or non-scattering state, and to scatter light and/or to absorb it especially when combined with an appropriate dye, in a relatively free or scattering state. An electric field may be selectively applied across the liquid crystals to switch between scattering and non-scattering states.
It is desirable that liquid crystal visual displays have excellent contrast between the characters displayed and the background, and high brightness in all ambient light conditions. It is also desirable that the display be free of front surface glare.
The present invention relates in a preferred embodiment described hereinafter to the use of a liquid crystal as a display medium that may be designated encapsulated operationally nematic liquid crystal material or nematic curvilinearly aligned phases ("NCAP") liquid crystal material.
A detailed explanation of operationally nematic or NCAP liquid crystal material is provided in U.S. Pat. No. 4,616,903 issued Oct. 14, 1986, in the name of Fergason, entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD, assigned to Manchester R&D Partnership, the disclosure of which is hereby incorporated by reference. Reference may also be made to U.S. Pat. No. 4,435,047, issued Mar. 6, 1984, in the name of Fergason, entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD, assigned to Manchester R&D Partnership, which disclosure is also hereby incorporated by reference.
In the field-off condition, or any other condition which results in the liquid crystal being in a distorted or randomly aligned state, the NCAP liquid crystal material scatters incident light. In the field-on condition, incident light is transmitted through the NCAP material.
A pleochroic dye may be present with the liquid crystal material to provide substantial attenuation by absorption in the field-off state but to be substantially transparent in the field-on state. Any reference to the ability of liquid crystal to scatter and/or absorb light in accordance with the present invention should not be limited to the scattering and minimal absorption properties of liquid crystal but should include the additional properties pleochroic dyes may impose on the optical properties of the liquid crystal.
The display medium of the display of the present invention may also comprise other scattering-type display materials, e.g., dynamic scattering liquid crystal systems or ferroelectric ceramic systems comprising optically clear (Pb,La)(Zr,Ti)(O.sub.3) ("PLZT"). The dynamic scattering and PLZT display mediums are both switchable between light scattering and non-scattering states.
In reflective liquid crystal displays utilized heretofore, the use of a gain reflector can produce higher brightness in the field-on state when the illumination on the display is collimated or quasi-collimated. However, in the field-off state, brightness is also increased, thereby providing little or no improvement in the contrast ratio. When the illumination on such displays is diffuse, the gain reflector will not affect the brightness at all.
The present invention relates to improvements in reflective displays utilizing a display medium variable between light scattering and non-scattering states. The present invention also relates to the use of the light scattering and absorption characteristics of NCAP liquid crystal materials. The invention further relates to the use of such liquid crystal materials and characteristics, together with a pleochroic or diochroic dye, for example, to obtain relatively high contrast and dark characters or information displayed on a relatively bright background in both small and large size displays.
An object of the present invention is to provide a display having a relatively high contrast as well as brightness.
A further object of the present invention is to provide a display that has excellent contrast and high brightness in all ambient light conditions.
Another object of the present invention is to improve the performance of a display in viewing conditions where glare is present.